Bowling balls are generally drilled to provide holes for two fingers and one hole for the thumb. The centers of these holes are preferably at the apices of an isosceles triangle, with the thumb hole centered on the intersection of the surface of the ball and a polar plane which is perpendicular to and bisects the line drawn between the centers of the finger holes.
The finger holes are usually drilled round and the fingers are comfortably received in them. Also, many bowlers prefer not to have finger holes at all, However, the thumb hole presents different problems, and a thumb hole is always used.
The bowler holds the ball with his thumb in the thumb hole with his four fingers divided in pairs, one pair on each side of the polar plane on which the thumb hole is centered. When finger holes are provided, one finger on each side goes into a respective finger hole. This means that the thumb will have to be rotated in order to reach its central position. If the thumb itself is round there is no problem. However, less than 10% of people have circular thumbs. Instead they are flattened and ovalled, so an oval hole is to be preferred for comfort and accuracy of throw. A hole which does not reasonably approximate the angular orientation and shape of the thumb will result in considerable physical irritation.
However, the angle which the thumb makes relative to the polar plane differs widely among people, so that an oval hole whose major axis is tilted at one angle may be suitable for one person and be completely unsuitable for another person. Plugging unsuitable holes and redrilling a ball is expensive, usually costing on the order of $20.00 each time. Customers understandably become impatient when the bowling professional drills too many unsuitable holes.
One problem with the most accurate known drilling procedure is that the cost of a machine tool to drill an oval hole at a precise angle is beyond the reach of many bowling pros. Instead a hand-held powered grinder is used, and for this the pro has little or no guidance to form a correctly oriented ovalled thumb hole.
It is an object of this invention to provide a gauge to determine the correct alignment of the major axis of an ovalled hole for each individual, and to provide a visible line to guide the pro showing the correct angle for the major axis so that even with a hand-held tool the pro can correctly align the hole he is drilling.